1. Field of the Invention
The invention relates to a cylinder head, and more particularly, to a cylinder head configured such that outlets of exhaust ports corresponding to a plurality of cylinders converge inside the cylinder head.
2. Description of Related Art
A cylinder head configured such that outlets of exhaust ports corresponding to a plurality of cylinders converge inside the cylinder head is known (see Japanese Patent Application Publication No. 2011-202578 (JP 2011-202578 A), for example).
JP 2011-202578 A describes a water jacket structure of a cylinder head provided with a plurality of cylinders, exhaust ports corresponding to the plurality of cylinders, and a water jacket provided near the exhaust ports.
In this cylinder head, the surface area of the exhaust, ports is increased by having the exhaust ports converge inside the cylinder head. As a result, heat exchange is promoted between exhaust gas that flows through the exhaust ports inside the cylinder head, and coolant that flows through the water jacket, which reduces the temperature of the exhaust gas (i.e., cools the exhaust gas).
Also, reducing the temperature of the exhaust gas in the exhaust ports reduces the heat load on a catalyst provided downstream of the exhaust ports, in an operating state in which the heat quantity of the exhaust gas increases, such as when the engine is under a high load or operating at a high speed.
With the cylinder head described in JP 2011-202578 A, having the exhaust ports converge inside the cylinder head results in the length of exhaust passages of the exhaust ports that extend from the cylinders positioned on the outside in a direction in which the cylinders are arranged (hereinafter, simply referred to as “cylinder arrangement direction”), becoming longer than the length of the exhaust passages of the exhaust ports that extend from the cylinders positioned on the inside in the cylinder arrangement direction. Therefore, the surface area of the exhaust ports with the longer exhaust passages that are positioned on the outside is larger than the surface area of the exhaust ports with the shorter exhaust passages that are positioned on the inside.
As a result, the temperature of the exhaust gas that flows through the exhaust ports with the longer exhaust passages that are positioned on the outside will be lower than the temperature of the exhaust gas that flows through the exhaust ports with the shorter exhaust passages that are positioned on the inside. Accordingly, a difference in the temperatures of the exhaust gas in the exhaust ports among cylinders increases. One conceivable way to eliminate this temperature difference is to reduce (i.e., not provide) a water jacket that is normally provided near the exhaust ports with the longer exhaust passages.
However, with this method, it is possible that the cooling effect on the engine oil that runs down inside the cylinder head may decrease by an amount corresponding to the reduction in the water jacket. That is, it may be difficult to reduce the temperature of the engine oil.